Full Idea
A mild version of the verification principle would say that it makes sense to think of someone as understanding an expression only if he is able, by his use of the expression, to give the best possible evidence that he understands it.
Gist of Idea
A milder claim is that understanding requires some evidence of that understanding
Source
Crispin Wright (Frege's Concept of Numbers as Objects [1983], 1.vii)
Book Reference
Wright,Crispin: 'Frege's Conception of Numbers' [Scots Philosophical Monographs 1983], p.44
A Reaction
That doesn't seem to tell us what understanding actually consists of, and may just be the truism that to demonstrate anything whatsoever will necessarily involve some evidence.